>perl -wMstrict -le
"my @lines = (
   'ABCD', 'A|B|C|D', 'A,B,C|D', 'A,B,C', 'A,B,C,D,E', 'A,B,C,D');
 ;;
 LINE:
 for my $line (@lines) {
   print qq{processing '$line'};
   ;;
   warn qq{'$line' has no comma} and next LINE
     if not $line =~ m{,}xms;
   warn qq{'$line' has nasty pipe} and next LINE
     if $line =~ m{ \| }xms;
   ;;
   warn qq{not exactly 4 fields in '$line'} and next LINE
     if 4 != (my ($s, $a, $c, $r) = split /,/, $line);
   ;;
   print qq{success: s '$s'  a '$a'  c '$c'  r '$r'};
   }
"
processing 'ABCD'
'ABCD' has no comma at -e line 1.
processing 'A|B|C|D'
'A|B|C|D' has no comma at -e line 1.
processing 'A,B,C|D'
'A,B,C|D' has nasty pipe at -e line 1.
processing 'A,B,C'
not exactly 4 fields in 'A,B,C' at -e line 1.
processing 'A,B,C,D,E'
not exactly 4 fields in 'A,B,C,D,E' at -e line 1.
processing 'A,B,C,D'
success: s 'A'  a 'B'  c 'C'  r 'D'